The invention relates to an apparatus for restoring a tight fitup provided against an adjacent structure and, more particularly, to a compact remotely installable jet pump piping support device that restores the tight and rigid fitup originally provided between the inlet mixer and the adjacent restrainer bracket in a boiling water nuclear reactor jet pump.
In a boiling water nuclear reactor (BWR), hollow tubular jet pumps positioned within the shroud annulus provide the required reactor core flow. The upper portion (inlet mixer) is laterally positioned and supported against two opposing rigid set screw contacts within restrainer brackets by a gravity actuated wedge, intended to assure clearances do not occur at the supports.
The flow through and outside the jet pumps contains pressure fluctuations from various sources in the reactor system. The pressure fluctuations can have frequencies close to one or more natural vibration modes of the jet pump piping, which depend on the tight fitup of the restrainer bracket contacts, which support the inlet mixer from the jet pump riser pipe.
Operating thermal gradients, component alignment changes, hydraulic loads, and fluctuations in these loads can overcome the lateral support provided by the gravity wedge. These effects and/or loosening of the set screws due to failure of their securing tack welds can cause clearances to develop at the opposing two fixed set screw contacts. This loss of contact can change the jet pump natural frequency to match some excitation frequency in the system, causing vibration of the piping and exerting increased loads which have caused cyclic fatigue cracking and wear of the supports and their welded attachments. This can result in degradation from wear and fatigue at additional jet pump structural supports, to a degree which may require plant shutdown.
The apparatus described herein is intended for BWR/6 reactor designs, where the available installation clearance access can be as small as 0.25 inches in width, and where disassembly of the inlet mixer to allow wedge installation is to be avoided.
Over the last 19 years, General Electric has designed and installed both gravity and spring actuated wedge supports at locations where clearances have developed in restrainer bracket contacts. The gravity wedges (see, e.g., U.S. Pat. No. 6,052,425) employed a tapered sliding wedge and a fixed bracket mount which engaged the jet pump restrainer bracket. The earlier types of these designs required disassembly of the jet pumps to allow access for their installation, which was an undesirable expense and extension of the reactor outage time. The gravity wedges were applicable to restrainer bracket/inlet mixer gap widths from about 1.0 to 2.0 inches, as space was required for a wedge with sufficient weight to give the required support load.
General Electric also implemented spiral spring actuated wedge repairs (see, e.g., U.S. Pat. No. 6,490,331) for BWR/4 reactor designs, where the available installation clearance access can be as small as 0.60 inches in width, and where disassembly of the inlet mixer to allow wedge installation was avoided. While it was initially intended that this prior design would also be installable in the smaller access space for BWR/6 reactors, without disassembly of the inlet mixers, this proved too difficult.
Another solution which had limited application was to reinforce the welded attachment of the two fixed contacts to the restrainer bracket, then reseat the inlet mixer against the fixed contacts when the jet pump was reassembled.